1. Field of the Invention
This invention relates to systems for promoting photocatalytic reactions. More particularly, the present invention relates to a reactor for causing photocatalytic reactions in a liquid, and then recapturing the photocatalyst by exploiting the upward buoyancy of the photocatalyst in relation to the liquid.
2. State of the Art
The value and importance of photocatalytic processes are well known, and research into new and better photocatalysts and their uses is ongoing throughout the world. There are many photocatalytic and photochemical mechanisms that have been found to be beneficial in a wide range of applications. For example, through photocatalysis, treatment of organic and inorganic contaminants in water is possible, management of chemical reagents within a process can be improved, and many chemical synthesis/manufacturing processes have been made possible which were either not possible or practical before.
Although the ability to promote photocatalytic reactions is well established, the feasibility of using these processes for practical water treatment, industrial processes, or chemical synthesis has been elusive due to the challenges associated with processing and handling finely dispersed semi-conductor photocatalyst materials in liquid. Use of such photocatalyst suspensions is often impractical because of the economic constraints of either sacrificing the photocatalyst (i.e. allowing disposal downstream) or recapturing it in difficult filtration and recycling steps.
Heretofore, photoreactors based on photocatalyst suspensions have difficulties providing both high oxidation efficiencies and acceptable processing flowrates while achieving economical management of the photocatalyst. In addition, some water treatment scenarios may mandate that the treated water be filtered prior to release to remove suspended solids in order to satisfy turbidity and clarity criteria.
It would thus be desirable to have an efficient photoreactor which both prevents the uneconomical loss of photocatalyst, while avoiding expensive and complicated filtration systems.
In concert with a photoreactor, some method of reflecting and concentrating light, typically sunlight, is generally desirable to promote higher efficiency in photochemical reactions. Although solar technologies in general have proliferated into niche areas in geographic regions where they are economically competitive, further research, development, refinement, and growth of these technologies is needed to make them more attractive and effective in locations that are not as amenable to solar technologies. One area that could benefit from additional development as that of solar collector/reflector devices. These devices act to capture solar energy through a large-surface mirrored area and reflect it back to a target area much smaller than the mirror. The net effect of solar collector and reflector devices is that solar energy may be concentrated far beyond the natural incident solar radiation and directed at a specific target. The inventor has found that there is a lack of such devices for vertically oriented targets (VOT""s), such as column-type reaction vessels, heat exchange pipes that depend upon thermosyphon operation, etc. Most prior solar reflector devices, such as horizontally oriented trough collectors, present limitations such as having a fixed reflective surface, and limited adjustability and mobility.
It would therefore be desirable to have a solar collector/reflector which is mobile and provides a reflective surface having an adjustable contour to accommodate the shape and dimensions of the target, and which is also angularly adjustable to optimize its position relative to the sun and the target.
It is therefore an advantage of the present invention to provide a photoreactor which manages and recycles a photocatalyst through control of hydraulic and buoyant forces.
It is another advantage of this invention to provide a photoreactor with a liquid-phase photocatalyst suspension which does not require downstream auxilliary equipment to recapture or filter out the photocatalyst.
It is another advantage of this invention to provide a photoreactor which allows good quantum efficiencies, while reducing the mass of photocatalyst which is required to promote photocatalytic reactions.
It is another advantage of this invention to provide a solar collector/reflector with an adjustable contour reflective surface to accommodate the shape and dimensions of a vertically oriented target.
The above and other advantages are realized in a system for the continuous use and recapture of a catalyst in liquid, comprising: a reactor having a reaction zone with generally downwardly flowing liquid, and a catalyst recovery chamber adjacent the reaction zone containing a catalyst consisting of buoyant particles. The liquid in the reaction zone flows downward at a rate which exceeds the speed of upward buoyant migration of catalyst particles in the liquid, whereby catalyst particles drawn out of the catalyst recovery chamber and introduced into the liquid in the reaction zone are drawn downward with the liquid. A flotation chamber disposed below the reaction zone is configured such that when the flowing liquid enters therein the liquid flow velocity drops, allowing the buoyancy of the catalyst particles to cause them to flow back into the catalyst recovery chamber, rather than being swept downstream.
In one illustrative embodiment, the photoreactor comprises a generally upright outer cylinder; with an inner cylinder disposed inside and generally coaxially with the outer cylinder, so as to define an annular reaction zone between the inner and outer cylinders. The inner cylinder comprises inlets at its bottom for allowing inflow of buoyant photocatalyst-coated microspheres, and an orifice disposed at its top for allowing outflow of the photocatalyst microspheres into the downwardly flowing liquid in the reaction zone. A flotation chamber is disposed at the bottom of the outer cylinder and is configured to substantially reduce the flow velocity of the liquid upon exiting the reaction zone, such that the buoyant microspheres are allowed to float upward through inlets of the inner cylinder. The flotation chamber may also include a means for forming a vortex in the liquid, such that the buoyant particles are further directed toward the inlets of the inner cylinder. A photoreflector is provided for directing light into the reaction zone, whereby the photocatalyst disposed on the microspheres induces desired chemical reactions within the liquid while flowing through the reaction zone.
The invention provides an economical and practical photoreactor which is useful for a wide variety of solar photocatalytic uses such as, for example, remediation of ground and surface waters, production of hydrogen and oxygen from water, recycling of costly chemical reagents such as chelating or complexing agents, as well as non-solar photocatalytic precesses which require specific wavelengths of light in concentrated form, such as, for example, chemical processes driven by lower UV wavelengths, such as less than 300 nm. Other advantages and features of the present invention will be apparent to those skilled in the art, based on the following description, taken in combination with the accompanying drawings.